It is known to form a wireless local area network in which data packets are direct sequence spread spectrum encoded and transmitted through air. Techniques for decoding such packets, and directed more specifically to the problem of synchronizing local PN codes with received data packets, are described, for example, in prior U.S. Pat. No. 4,774,715 to Messenger, one of the co-inventors of the present invention. The packets transmitted in such systems include a synchronization header that permits a receiving station to lock onto a received packet and then spread spectrum decode the data following the header. Without a synchronization header of adequate length, the data is apt not to be decoded in a proper manner.
One shortcoming associated with such prior systems relates to the length of the synchronization header and time required to synchronize to a received packet before data can be decoded. The size of the synchronization header ultimately limits data transfer rates. This is most significant where a communications channel is time-shared by a large number of users. A requirement for large synchronization headers can make such systems impractical for digitized voice transmission.
It is common in decoding spread spectrum transmissions to select different phase values for the PN code of a receiving station from a relatively exhaustive set of values. Such a set will correspond in size to the number of "chips" contained in the PN code. Phase-shifted PN codes corresponding to each of the phase values are produced and combined with incoming signals to determine a phase value that will synchronize the station's PN code with an intended communication. The strength of the test signals may be detected in a known manner to determine whether any selected phase value synchronizes the station's PN code with the received data. Thereafter, the data is simply decoded by combining the data with the PN code of the station adjusted to the synchronizing phase value.
One method of producing more rapid synchronization is to provide a measure of parallelism in the search for a synchronizing phase value. More specifically, sets of possible phase values may be simultaneously tested to determine whether any one of the phase values is appropriate for decoding the incoming signal. If repeated selection and testing of phase values is to be entirely eliminated, the required phase-shifting and combining circuitry must be duplicated until all practical phase values of the receiving station's PN code are simultaneously tested. Given the typical chip size of a PN code (in excess of 100 chips), providing parallel processing circuitry at various stations in a local area network would be exceptionally costly. Other rapid synchronizing techniques, including matched filters, are known, but these too are very costly